In my proposed book manuscript, I will trace how Jane Addams (1860-1935) developed her theory of cosmopolitan pluralism through engaging with a range of international theorists and working in a variety of transnational contexts. Best known for her work with immigrants at Hull House in Chicago and her leadership in progressive era social reforms, and for receiving the Nobel Peace Prize in 1931, Addams was also a prolific writer, producing ten books and hundreds of articles. My project lies at the intersection of intellectual history and philosophy. Working as intellectual historian, I will locate Addams's thinking among the astonishing array of international interlocutors with whom she shaped her conception of cosmopolitan pluralism. As philosopher, I will explore the inner workings of Addams's theorizing and show how Addams's conception of cosmopolitan pluralism was an integral component of her theories of social democracy and pacifism. Neither task has been done to date.